1. Field of the Invention
The present invention relates to a sintered aluminum nitride used as insulated boards of circuit boards or semiconductor packages. More particularly, the invention relates to a sintered aluminum nitride which can be fired at a low temperature and exhibits excellent resistance against chemicals and favorable thermal conductivity.
2. Description of the Prior Art
In recent years, known semiconductor devices are generating more heat accompanying a trend toward highly densely integrating the semiconductor elements. In order to eliminate malfunction of such semiconductor devices, it has become necessary to use a board which radiates heat out of these devices. However, an alumina material that has heretofore been used for the boards has a thermal conductivity which is as small as about 20 W/m.cndot.K. Attention, therefore, has been given to aluminum nitride having a high thermal conductivity. In its single crystalline form, the aluminum nitride has a theoretical thermal conductivity of as large as 320 W/m.cndot.K. In recent years, a sintered aluminum nitride has been developed having a thermal conductivity of not smaller than 200 W/m.cndot.K.
Such a sintered aluminum nitride is prepared by firing at a temperature of not lower than 1800.degree. C. by adding, as a sintering assistant, a rare earth element compound (Japanese Patent Publication No. 46032/1988) or an alkaline earth element compound (Japanese Patent Publication No. 49510/1983, Japanese Laid-Open Patent Publication No. 10071/1986).
According to this method, however, the cost of production increases due to wear of jigs used for the firing, structure of the firing furnace and running costs. In order to decrease the cost of production, therefore, a firing method has been proposed at a temperature of as low as 1600.degree. to 1700.degree. C. by simultaneously adding a rare earth element compound and an alkaline earth element compound as sintering assistants (Japanese Patent Publications Nos. 49613/1994 and 17454/1995).
In producing the products such as packages by using a sintered product obtained by simultaneously adding a rare earth element compound and an alkaline earth element compound thereto, however, the sintered product is deteriorated in the step of plating due to chemicals such as acids and alkalis permitting its surfaces to be coarsened or discolored. In particular, the aluminum nitride itself dissolves in an alkaline chemical and is deteriorated to a conspicuous degree. Due to the firing at a low temperature, furthermore, the color tone on the surface of the sintered products tends to be whitened.